There have been previously proposed various kinds of time/frequency measuring apparatus for continuously measuring various parameters in the domain of time/frequency of a pulse signal to be measured, for example, period, frequency, pulse width, duty ratio, time interval, phase or the like of the pulse signal and analyzing a pulse width fluctuation, namely, a jitter, of the pulse signal to be measured. However, the prior art measuring apparatus have problems that the accuracy of measurement is low, the circuit composition or construction is complicated, the troublesome control is required, etc.
To this end, the assignee of the present application filed a patent application for an invention entitled "jitter analyzer" for analyzing a jitter contained in an input pulse (see Japanese Patent Application Public Disclosure No. 107287/1993 or U.S. Pat. No. 5,293,520). The jitter analyzer disclosed therein is a kind of time interval analyzer for calculating a period and a pulse width of a pulse signal inputted as a signal to be measured as well as analyzing a component of a jitter contained in the pulse width by effecting the fast Fourier transform (hereinafter also referred to as FFT) of the calculated data of pulse width and analyzing the results. As to the detailed explanation thereof, see the above-mentioned Japanese Patent Application Public Disclosure No. 107287/1993 or U.S. Pat. No. 5,293,520.
Also, the assignee of the present application filed a utility model application for an invention entitled "an apparatus for continuously measuring pulse widths" on Apr. 28, 1993 (Japanese Utility Model Application No. 27920/1993). The pulse width measuring apparatus disclosed therein can continuously measure pulse widths of input pulses without being restricted by the duty ratios thereof and make higher the limitation of measurable frequency.
Recently, digital communications have been utilized by persons and as a result communications utilizing a discontinuous burst signal as represented by a TDMA (time division multiple access) communication system have prospered. Therefore, it has become important to analyze various parameters in the domain of time/frequency of the burst signal, that is, a jitter contained in the burst signal.
In digital communications or digital recording/reproducing apparatus (magnetic, optical or the like), information is divided in time within a predetermined section or segment and stored. Hence it becomes important to measure only a section of data among a stream of data or to inhibit from measuring only a section of data among a stream of data. For example, in TDMA communication system, information transmitting slots appear in a burst-like manner and so it is necessary to detect a section or segment having no desired data therein and to start to measure immediately after the detected section has been terminated. On the other hand, in an optical or magnetic recording apparatus, there may be cases in which a header information of a sector is intended to be excluded from the measurement and a section of the sector which has only a user information field or fields therein is intended to be measured.
In digital communication networks as represented by an architecture such as ISDN (integrated services digital network), SONET (synchronous optical network)/SDH (synchronous digital hierarchy), etc., a jitter and/or a wander is considered to be a main factor which prevents from implementing or effecting a high-speed transmission of data with high quality and large capacity. The jitter is prescribed in its amplitude value and frequency component according to the advice of ITU/TS (formerly CCITT) and the like, and the wander is prescribed in the maximum value of MTIE (maximum time interval error) for an observation time.
Generally, in a transmission communicating system, a PLL (phase locked loop) of a relatively low gain is used to reproduce clocks received. Accordingly, a jitter of high frequency component is suppressed to a certain degree by the PLL, but a jitter of low frequency component and/or a wander which cannot be fully removed by the PLL raises a problem in actual use.
Further, in SONET/SDH which is important as a social foundation in future, there is provided a function of insertion and extraction of a pointer existing in the heading portion of a transmission frame in order to maintain the synchronization in a network interface, and also, in ATM (asynchronous transfer mode) there is provided a function by which a reference frequency information of an originating side is transferred to a receiving side by use of a time stamp and the receiving side reproduces the reference frequency information of the originating side. These new synchronizing techniques cause a further problem that a jitter of lower frequency (1 .mu.Hz-10 Hz) and a wander are produced thereby, and it is apprehensive that a jitter is more and more lowered in frequency thereof.
The above-mentioned prior time interval analyzer is aimed principally at analyzing a continuous signal and so in case that a discontinuous burst signal is inputted into such analyzer as a signal to be measured in order to measure various parameters in the domain of time/frequency thereof, such parameters are continuously measured by the analyzer irrespective of on/off (existence/nonexistence) of the burst signal. Therefore, the necessary data during an on-interval of the burst signal (a time interval that the burst signal exists) and the unnecessary data during an off-interval of the burst signal (a time interval that the burst signal does not exist) are mixed together in the result of measurement obtained, and the analysis of the obtained data is prevented thereby. Particularly, in the analysis of a variation for the time elapsed and frequency, the obtained data including the unnecessary data during off-intervals of the burst signal are processed all together, and so there is a drawback that the desired result of analysis is not obtained.
In order to remove this prior art drawback, there are proposed some time interval analyzers having an input terminal for coping with a burst signal to which a measurement inhibiting signal is inputted. That is, each of such time interval analyzers is provided with "a measurement inhibiting signal input" as an external input terminal for measuring a burst signal, and the measurement inhibiting signal input receives a measurement inhibiting signal (which is generated during an off-interval of the burst signal) transmitted together with the burst signal which is to be measured thereby causing the analyzer to stop the measurement thereby during the off-interval of the burst signal. As a result, the measurement by the analyzer is performed only during the on-interval of the burst signal, namely, only when the measurement inhibiting signal is not received on the measurement inhibiting signal input. Therefore, in a statistical operation and an analysis of distribution of the number of occurrences the result of measurements obtained only during the on-intervals of the burst signal is processed, and the unnecessary data during the off-intervals of the burst signal are not contained in that analysis. Also, each such analyzer is constituted such that marks each indicating a measurement inhibiting interval are displayed on a graph in an analysis of a variation for the time elapsed or the like.
However, in such prior art analyzers thus constructed, in addition to a signal to be measured, a measurement inhibiting signal (a burst on/off signal synchronized with the existence and nonexistence of a burst signal) is always necessary. In general environment of measurement there are many cases that only a signal to be measured is transmitted/received and a burst on/off signal is not transmitted/received, and as a result there is a drawback that a condition for measurement is restricted. In addition, since data during measurement inhibiting intervals are not acquired, this results in an essential defect that an analysis of frequency by FFT cannot be effected.
Also, the above-mentioned prior aft time/frequency measuring apparatus, inclusive of the measuring apparatus described in the above-mentioned Japanese Patent Application filed by the assignee of the present application, have the difficulty of analyzing a jitter contained in a burst signal, and an external trigger input function by which a measurement is started after an external timing signal has been inputted and an edge of level (low/high) of this timing signal has been acquired, a delay trigger function by which a measurement is started after a time interval preset by a user has elapsed or the number of edges of a signal to be measured preset by a user has occurred, of the like is used by the prior aft apparatus as a trigger function thereof. Accordingly, it is impossible by such prior aft trigger functions to perform the measurements in which a specified section of segment in a signal to be measured is detected and data measured is started to be stored immediately after the detection of the specified section of data measured is inhibited from being stored immediately after the detection of the specified section.
Further, there is proposed a time interval measuring apparatus with a time window function which is able to acquire a specified data in a signal to be measured and is disclosed, for example, in U.S. Pat. No. 4,611,926. However, the constitution and control procedure of the time window circuit disclosed in the U.S. Pat. No. 4,611,926 are aimed at realizing a discriminating function of measured values and only when data within a range set by a user is observed, the data is adopted as a measured value.
That is, because this time window circuit is equivalent to the circuit constitution and control procedure which are aimed at only acquiring data within a range from a stream of data, it is also difficult for such time window circuit to detect a specified section or segment in the data stream and to start to store data measured immediately after the detection of the specified section or to inhibit from storing data measured immediately after the detection of the specified section.
It is said hitherto that a digital transmission analyzer is capable of analyzing a jitter contained in a pulse-like signal to be measured. This digital transmission analyzer is a tester which measures a transmission error and a jitter, and mainly comprises a transmitting part and a receiving part. The receiving part has a jitter measurable function, but parameters measured by this analyzer are restricted mainly to an amplitude of jitter and a frequency or number of occurrences of data over a certain amplitude value of jitter, and it is said that this analyzer has not a function of analyzing a frequency component. Because in case of analyzing a frequency component of a jitter in this digital transmission analyzer, there is no other way that a jitter frequency of an output signal is set by the transmitting part and the jitter amplitude at that time is measured in the receiving part, and therefore, it is impossible to effect an analysis of continuous jitter frequency components ranging from a low frequency to a high frequency. In addition, the lower limit of a region of setting a jitter frequency is of the order of 10 Hz-100 Hz which is an insufficient value to analyze a jitter of a low frequency, and a wander cannot be measured.
On the other hand, prior art SONET/SDH analyzers publicly known, generally, are aimed at mainly effecting function tests, and some of them are deemed to be ones which can analyze a jitter/wander. Such type of the analyzers comprise a transmitting part and a receiving part separated from each other similar to the above-mentioned digital transmission analyzer, and a frequency and amplitude of a jitter are set by the transmitting part and the jitter amplitude is measured in the receiving part. In addition, the lower limit of a region of setting a jitter frequency is as low as 1 Hz which makes it possible to effect an analysis of jitter frequency to 1 Hz, but this lower limit of 1 Hz is still insufficient. Also, such analyzers have a function capable of measuring a wander in an observation interval of 100 msec (0.1 sec) to 100 sec. However, this observation interval is merely a little bit of the scope of observation interval according to the advice of ITU/TS and it is extremely insufficient.
The observation interval of MTIE manifested by the advice of ITU/TS ranges from 50 msec to 10.sup.9 sec and this observation interval is divided by two ranges, one being the range from 50 msec to 500 sec, the other being the range from 500 sec to 10.sup.9 sec. Among them the range from 50 msec to 10.sup.7 sec is regarded as important for a wander in view of realization of the observation interval (the advice G. 811). Accordingly, it is impossible by the above-mentioned observation interval from 0.1 sec to 100 sec to measure a wander with a high accuracy.